The present invention relates to methods for characterizing the plugging efficiency of drilling fluids and related compositions and methods of making and using such drilling fluids.
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit and providing a hydrostatic head to maintain the integrity of the wellbore walls, and prevent well blowouts. Specific drilling fluid systems, which can be oil-based or aqueous-based, are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Treatment fluids often contain additives to adjust the characteristics of the fluid, i.e., wettability, viscosity; to provide for a specific treatment, i.e., lost circulation control or fracturing; in an effort to efficiently and effectively treat a subterranean formation. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid.
One of the issues facing drillers today is lost circulation. Lost circulation is loss of a drilling fluid into a subterranean formation through natural or induced voids. Lost circulation leads to significant loss in time and increase in costs due to the loss of whole drilling fluid, wasted rig time, and in the worst cases may lead to lost wellbores, side tracked or bypassed wells, abandoned wells, relief wells, and lost petroleum reserves. This is especially a problem in formations that have zones that have large voids.
Particulates, a common additive in drilling fluids, can help modulate the flow of fluids within, into, and out of the subterranean formation and/or the wellbore. More specifically, particulates may be used in drilling fluids to bridge fractures, provide fluid loss control, provide lost circulation control, seal surfaces for fluid diversion, plug voids, prop open fractures, and any combination thereof. The types and concentration of particulates added to a drilling fluid greatly impact the efficacy and efficiency of a given treatment. For example, the size and shape of the particulates can help determine whether the particulates allow or retard fluid flow. For example, proppants are designed to be substantially spherical so as to allow fluid flow around the particle as it holds open a void. Fibers have been of interest as particulate additives, but fibers may lead to problems like high viscosity and plugging downhole tools before reaching the formation.
Particulate concentration in a drilling fluid is estimated based on experiences with implementation in similar subterranean formations. As new particulates, e.g., lost circulation materials, and drilling fluids become available, a need exists to quickly determine an appropriate concentration of particulate to include in a treatment fluid to provide the appropriate level of fluid flow within, into, and out of a subterranean formation and/or wellbore.